MRI is an imaging method which magnetically excites nuclear spin of an object set in a static magnetic field with a RF (radio frequency) signal having the Larmor frequency and reconstructs an image based on NMR (nuclear magnetic resonance) signals generated due to the excitation.
A static magnetic field magnet included in an MRI apparatus generates a static magnetic field whose intensity is high for imaging. For this reason, in order to avoid magnetic influence on surrounding electronic devices, an MRI apparatus is placed in an imaging room which has a function as a shield room to magnetism.
In a recent MRI apparatus, a super conductive magnet is used as a static magnetic field magnet. Therefore, it is possible to form a high magnetic field more than or equal to 1.5 [T] (tesla) in an imaging space. On the other hand, an old MRI apparatus has a permanent magnet as a static magnetic field magnet and can form a magnetic field more than or equal to 0.3 [T]. A comparatively recent MRI apparatus can form an about 0.8 [T] of magnetic field.
In recent years, the needs of replacing an old type of MRI apparatus, which passed through long years, with a newest MRI apparatus are growing. However, a shield room for an outdated MRI apparatus having a permanent magnet is designed so that a sufficient magnetic shield effect may be gained against a 0.3 [T] to at most 0.8 [T] of magnetic field. For this reason, if an MRI apparatus which forms a high magnetic field of 1.5 [T] and above is placed in a conventional and narrow shield room as it is, a problem of nonnegligible magnetic field leak from the shield room arises.
In order to solve this problem, ingenuity for placing a piece of iron on the wall of the shield room in the side near a gantry in which a static magnetic field magnet is built is made. Specifically, a thickness and the like of a piece of iron are adjusted so that isomagnetics of 5 [G] (gauss; 1 [G]=1×10−4 [T]) do not leak from a shield room when an MRI apparatus having a super conductive magnet is placed in the shield room. Thereby, it becomes possible to place an MRI apparatus having a super conductive magnet in a shield room for an outdated MRI apparatus having a permanent magnet.